The present invention relates to a method for abrading both faces of a work piece.
A lapping machine is one type of abrasive machine for abrading thin work pieces, e.g., silicon wafers.
In the lapping machine, a carrier holding work pieces is sandwiched between an upper abrasive plate and a lower abrasive plate which are rotated in opposite directions. The carrier is driven by a sun gear and an internal gear so that the work pieces are rotated and moved along a circular orbit. With this action, both faces of the work pieces can be lapped by the abrasive plates. The upper abrasive plate is vertically moved by a rod of a cylinder unit. When abrasion is started, the upper abrasive plate is slightly suspended by the cylinder unit so as not to apply the full weight of the upper abrasive plate. Suddenly applying a great force to the work pieces can thus be prevented during an initial abrasion step. This is called xe2x80x9clow pressure abrasionxe2x80x9d. After the initial abrasion step, the full weight of the upper abrasive plate is applied to lap and finish the work pieces.
A similar structure is employed in a polishing machine. In the polishing machine, polishing cloth is adhered on an abrasive face of each abrasive plate to polish both faces of the work pieces.
In the conventional abrasive machine, e.g., the lapping machine, the polishing machine, the full weight of the upper abrasive plate is applied during a main abrasion step.
By applying the full weight of the upper abrasive plate, deformation of the heavy upper abrasive plate can be prevented and both faces of the work pieces, e.g., silicon wafers for semiconductor devices, can be made highly flat.
In the case of lapping silicon wafers, for example, preferred pressure for lapping the wafers is 100-120 g/cm or 9.8-1 1.76xc3x9710 Pa. Therefore, in the case of applying the full weight of the upper abrasive plate, the weight and thickness of the upper abrasive plate must be limited.
However, the upper abrasive plate is gradually abraded and its weight is also varied. For example, the weight of the upper abrasive weight is reduced from 500 kg to 495-490 kg in a week.
Reducing the weight of the upper abrasive plate adversely influences abrasive rate so that the time for abrading the work pieces gradually increases. If the weight of the upper abrasive plate is varied, abrasive conditions must be changed every time, and quality of products are not fixed. Since the time for abrading the work pieces must be longer, working efficiency must be lower.
The present invention is solves the disadvantages of the conventional method.
An object of the present invention is to provide a method for abrading both faces of a work piece in which the work piece can be abraded with a fixed load.
To achieve the object, the present invention has the following components.
Namely, the method of the present invention is performed in an abrasive machine including:
a rotatable lower abrasive plate;
a rotatable upper abrasive plate being provided to face the lower abrasive plate to clamp the work piece with the lower abrasive plate; and
a cylinder unit having a rod from which the upper abrasive plate is suspended, the cylinder unit moving the upper abrasive plate in a vertical direction.
Using these components, a method comprises:
a first abrading process in which pressure of a cylinder chamber of the cylinder unit is adjusted to apply a first pressure to the work piece via the upper abrasive plate without applying the full weight of the upper abrasive plate; and
a second abrading process in which the pressure of the cylinder chamber is readjusted to apply a second pressure higher than the first pressure to the work piece via the upper abrasive plate without applying the full weight of the upper abrasive plate.
The method may further comprise a third abrading process in which the pressure of the cylinder chamber is readjusted to apply a third pressure lower than the second pressure to the work piece via the upper abrasive plate.
In the method, a reinforcing rib may be provided to an upper face of the upper abrasive plate to increase the rigidity thereof.
In the method, a holding disk may be fixed to the rod of the cylinder unit, a plurality of connecting rods may be provided to the holding disk, and the upper abrasive plate may be fixed to the connecting rods.
By employing the reinforced upper abrasive plate or holding the upper abrasive plate with the connecting rods, the deformation of the upper abrasive plate, which is caused by its own weight, can be prevented even if the upper abrasive plate is always suspended for the xe2x80x9clow pressure abrasionxe2x80x9d. Therefore, both faces of the work piece can be made highly flat.
A heavy and thick upper abrasive plate may be used to increase the rigidity thereof. In this case, the xe2x80x9clow pressure abrasionxe2x80x9d is performed so a fixed load or pressure can be applied.
Further, in the method, the first abrading process and the second abrading process may be performed by:
calculating the constant xe2x80x9cAxe2x80x9d, on the basis of: a formula xe2x80x9cW=xe2x88x92Axc2x7P+Bxe2x80x9d (B: the weight of the upper abrasive plate, P: the total pressure of the cylinder chamber of the cylinder unit, A: a proportional constant relating to frictional loss, etc., W: actual load applied from the upper abrasive plate); the known weight xe2x80x9cB1xe2x80x9d of the upper abrasive plate; the measured actual load xe2x80x9cW1xe2x80x9d applied from the upper abrasive plate when an optional load is applied to the upper abrasive plate; and the measured total pressure xe2x80x9cP1xe2x80x9d of the cylinder chamber;
calculating a value xe2x80x9cP2xe2x80x9d which satisfies a formula xe2x80x9cW2=xe2x88x92Axc2x7P2+B1xe2x80x9d (W2: the set actual load applied from the upper abrasive plate while abrading);
monitoring the pressure in the cylinder chamber; and
adjusting the total pressure in the cylinder chamber to the value xe2x80x9cP2xe2x80x9d, and
the first abrading process and the second abrading process of the subsequent time in which another work piece is abraded a prescribed amount may be performed by:
calculating the value xe2x80x9cB1xe2x80x9d (=A Px) of a balanced state on the basis of the value xe2x80x9cWxe2x80x9d (=0) and the measured total pressure xe2x80x9cPxxe2x80x9d of the cylinder chamber;
replacing the value xe2x80x9cB1xe2x80x9d with the value xe2x80x9cPxxe2x80x9d; and
calculating a value xe2x80x9cP3xe2x80x9d which satisfies a formula of xe2x80x9cW3=xe2x88x92Axc2x7P3+B1xe2x80x9d (W3: the set actual load applied from the upper abrasive plate while abrading);
monitoring the pressure in the cylinder chamber; and
adjusting the total pressure in the cylinder-chamber to the value xe2x80x9cP3xe2x80x9d. In this case, the fixed pressure can be easily set every time by simple calculation so the work pieces can be uniformly abraded every time.
In the method of the present invention, the xe2x80x9clow pressure abrasionxe2x80x9d can be performed throughout the abrasion so abrasion of the upper abrasive plate does not adversely influence the quality of products. Further, the fixed pressure can be always applied so that the work pieces can be uniformly abraded every time. The abrading conditions can be easily set.